BETA-CGRP, HUMAN

Gene, mRNA, and precursor

The human CGRP-α gene (CALCA), located on chromosome 11 (11p15.2), consists of six exons. CGRP-α mRNA is synthesized by the alternative splicing of exons 1, 2, 3, 5, and 6 of the CALCA gene. Exon 4 codes for calcitonin (CT). The human CGRP-β gene (CALCB), located in the vicinity of CALCA on chromosome 11 (11p15.2), consists of five exons. CALCB encodes only CGRP-β and does not contain a CT-like sequence. The sequence and structure of CALCA are well conserved among all vertebrate species. CALCB is preserved in primates and in rodents. In fish, the medaka also possesses two types of CGRP gene (cgrp1 and cgrp2), but medaka cgrp2 is not the ortholog of mammalian CALCB. Mammalian CALCA and CALCB are considered to have been generated by tandem duplication that occurred later than the separation of the Euarchontoglires lineages, for they are located in the same chromosome. In contrast, medaka cgrp genes are located in separate chromosomes, indicating that they were produced by the teleost-specific whole genome duplication that occurred after the separation of the teleost lineage.

Receptors

The functional CGRP receptor is derived from the calcitonin receptor-like receptor (CLR), whose phenotype is determined by coexpression with the receptor activitymodifying protein (RAMP). The coexpression of CLR with RAMP1 results in the CGRP receptor, whereas coexpression with RAMP2 or RAMP3 produces the receptor for adrenomedullin, another member of the CGRP family. CLR is a seven-transmembrane-domain GPCR consisting of 474–548 aa residues in mammals; it shares 55% sequence identity with the CT receptor. RAMP is a single-transmembrane accessory protein that regulates the activities of several GPCRs. Besides contributing to receptor specificity, RAMPs are required for the transportation of CLRs from the endoplasmic reticulum to the plasma membrane. Three types of RAMPs consisting of 148–175 aa residues exist in mammals, and five types are identified in teleost fish. The functional CGRP receptor requires another accessory protein, the receptor component protein (RCP). The CT receptor (CTR)/RAMP1 complex, which is known as the amylin receptor, is also reported to be responsive to CGRP.

Agonists and Antagonists

Recombinant human CGRP-α, amylin. CGRP8–37 fragment, BIBN4094BS, MK3207, BI44370A, SB-273779, WO98/11128, and BMS-927711.

Biological functions

CGRP exerts vasodilation in the microvasculature in the brain, heart, gastrointestinal system, limbs, and skin. CGRP, released from the terminal of sensory nerves, binds its receptor in the vascular smooth muscle cells and causes vasodilation. CGRP also has a cardioprotective role against ischemia/reperfusion injury. CGRP is likely to be involved in various biological actions; its plasma level increases during pregnancy in rats, it inhibits gastric acid secretion in rats and dogs, and the gene transfer of CGRP restores erectile function in aged rats.

Clinical implications

CGRP is actively studied in relation to migraines. The jugular level of CGRP is increased during migraine attacks, and intravenous CGRP administration induces a migraine-like headache in humans. CGRP receptor antagonists are considered to be effective for the treatment of migraines. Increased plasma CGRP levels are also observed in myocardial ischemia and sepsis. CGRP may be implicated in Raynaud’s disease, the syndrome characterized by severe peripheral vasospasms. The administration of CGRP leads to peripheral vasodilation and promotes the healing of ulcers in patients with this disease.

Synthesis and release

CGRP is synthesized in a variety of central and peripheral neurons, and is packaged into vesicles that are transported to nerve terminals. CGRP release from sensory nerve terminals is stimulated by vascular wall tension, steroids such as estrogens and androgens, bradykinin/ prostaglandins, and endothelin. During a migraine, the activation of trigeminal nerves stimulated by inflammatory substances from the cerebrovasculature leads to the release of CGRP. In the gastrointestinal system, the synthesis and release of CGRP are triggered by the activation of the capsaicin receptor TRPV1 in the sensory nerves. However, the exact mechanism of CGRP release is largely unknown.

Description

CGRP is a vasodilative neuropeptide synthesized mainly in the central nervous system. It is a transmitter of nociceptive signals in relation to migraines. The presence of CGRP-α (or αCGRP) mRNA formed by alternative splicing from the calcitonin gene (CALCA) was reported in 1982, and it was isolated in 1984 from human medullary thyroid calcinoma. CGRP-β (or βCGRP) was identified by human genome analysis in 1985.

Structure and conformation

Two types of CGRP (CGRP-α and CGRP-β) are conserved among several mammalian species in Euarchontoglires. Human proCGRP-α and mature CGRP-α consist of 128 aa and 37 aa residues, respectively. Human proCGRP-β and mature CGRP-β consist of 127 aa and 37 aa residues, respectively. The sequence identity between CGRP-α and CGRP-β is 92% in the human and 97% in the rat. The sequence of CGRP-α is highly conserved among vertebrates from fish to mammals. Glycine-14 and leucine-15 sequences are conserved in mammalian CGRP-α peptides, whereas aspartic acid and phenylalanine are conserved in the region of nonmammalian CGRPs. CGRP-α and CGRP-β both contain a ring structure formed by the single disulfide bond between the cysteine-2 and cysteine-7 residues. The carboxyl end is amidated. CGRP-α, Mr. 3789.4; CGRP-β, Mr. 3793.4. pI 5.3. Soluble in water and physiological saline solution.